Flue gas desulfurization with ammonium sulfite

ABSTRACT

Flue gas is desulfurized by absorption in aqueous ammonium sulfite in a multiple-stage countercurrent absorber. The absorber effluent solution is regenerated by acidifying a portion thereof with ammonium bisulfate to liberate sulfur dioxide decomposing the resulting ammonium sulfate at high temperature by direct contact with hot combustion gases, and by reacting the ammonia thus formed with a second portion of the absorber effluent solution to prepare fresh ammonium sulfite absorbent solution. Regeneration is carried out at constant rate regardless of variations in the flow rates of flue gas and absorbent solution in the absorber; this is accomplished by use of surge tanks for storage of both absorbent and absorber effluent solutions.

United States Patent Griffin, Jr. et al.

[ Feb.29, 1972 [54] FLUE GAS DESULFURIZATION WITH AMMONIUM SULFITE [72]Inventors: Lindsay 1. Grtllin, Jr., Summit; Albert 11.

Welly, .lr., Westficld, both of NJ.

[52] US. Cl. ..23/2 SQ, 23/178 [51] llt.Cl..... ..C0lb 17/60,C0lc 1/22[58] Fieltlolsearch ..23/2, 178, 193

[56] References Cited UNITED STATES PATENTS 2,021,936 11/1935 .lohnstone..23/2 X 2,676,090 4/ 1954 .lohnstone .2312 X 2,405,747 8/1946 Hixson etal. .23/2 X 3,275,407 9/1966 Furkert et al. ..23/178 X 3,321,275 5/1967Furkert et al. .l...23/178 OTHER PUBLICATIONS Slack, A. V., ChemicalEngineering" December 4, 1967, pp. 191- 92. I

Primary Examiner-Oscar R. Vertiz Assistant Examiner-G. O. PetersAttorney-Pearlman and Stahl and Louis F. Kreek, Jr.

[57] ABSTRACT Flue gas is desuliurized by absorption in aqueous ammoniumsulfite in a multiple-stage countercurrent absorber. The abso'rbereffluent solution is regenerated by acidifying a portion thereof withammonium bisulfate to liberate sulfur dioxide decomposing the resultingammonium sulfate at high temperature by direct contact with hotcombustion gases, and by reacting the ammonia thus formed with a secondportion of the absorber effluent solution to prepare fresh ammoniumsultite' absorbent solution. Regeneration is carried out at constantrate regardless of variations in the flow rates of flue gas andabsorbent solution in the absorber; this is accomplished by use of surgetanks for storage of both absorbent and absorber effluent solutions.

8 Claims, 1 Drawing Figure FLUE GAS DESULFURIZATION WITH AMMONIUMSULFITE BACKGROUND OF THE INVENTION tion of the atmosphere.

Various processes have been suggested for removal of sulfur dioxide fromflue gas, although none has gained a general in- .dustry acceptance todate. These processes may be grouped generally as wet processes and dryprocesses. Wet processes are those which employ an absorbent solution,usually aqueous, for removal of sulfur dioxide from a gas stream.

A flue gas desulfurization process has several requirements. First, itmust be capable of removing most of the sulfur dioxide content of theflue gas, preferably 90 percent or more of the S present, under widelyvarying load conditions. Second, it should not create any air or waterpollution problems. Third, the process should be easy to operate andmaintain. The process should have a low net cost. In many instances thiswould require the production of a salable byproduct. The process shouldbe capable of incorporation into existing power plants if it is toachieve maximum application. This requirement favors wet processes,which operate at a low temperature and therefore can be placed after theconventional air preheater in which incoming air for combustion isheated by the hot flue gas. Dry processes usually require a much higheroperating temperature, and therefore must be inserted ahead of thepreheater and integrated with the power plant.

Various wet processes have been described in the art, as, for example,those described in Hixson et al., U.S. Pat. No. 2,405,747, issued Aug.l3, i946, and .lohnstone et al., U.S. Pat. No. 2,134,48l, issued Oct.25, 1938. Hixson describes the use of aqueous ammonia as the absorbent..lohnstone et al. uses aqueous ammonium sulfite as the absorbent, andregenerates the scrubber effluent by boiling.

The present invention provides a process which will effectively removesulfur dioxide from flue gas under widely varying load conditions, whichcan be installed in existing power plants, and which is simple andeconomical to operate.

SUMMARY OF THE INVENTION According to the present invention, sulfurdioxide is removed from flue gas by contacting a flue gas stream in anabsorber with an aqueous absorbent solution which is principallyammonium sulfite. A desulfurized flue gas stream and an effluentsolution which is principally ammonium bisulfite are withdrawn from theabsorber. The absorber effluent is divided into two portions, and oneportion is reacted with ammonium bisulfate to liberate sulfur dioxide.The resulting ammonium sulfate, which is in the form of an aqueousslurry, is contacted with hot combustion gases in a decomposer to formmolten ammonium bisulfate and ammonia gas. The ammonium bisulfate isreturned to the aeidifier. The ammonia gas is contacted with the secondportion of the ammonium bisulfite effluent solution from the absorber inorder to form the ammonium sulflte containing absorbent solution.

THE DRAWING This invention will now be described in further detail withreference to the accompanying drawing, in which the sole FIGURE is aflow sheet of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT This process is generallyapplicable to treatment of waste gas streams containing sulfur dioxideas an undesired impurity. This invention is especially useful intreating flue gas streams which are formed by combustion of fossilfuels, i.e., coal and oil, which contain sulfur. Such flue gas streamsgenerally contain up to 0.3 percent by volume, of sulfur dioxide, somefree oxygen due to the use of excess combustion air, and small amountsoffly ash.

The process of this invention includes the steps of (l) absorbing sulfurdioxide in an aqueous absorbent solution which is primarily ammoniumsulfite; (2) acidifying a portion of the absorber effluent solution toliberate sulfur dioxide; (3)

decomposing the ammonium sulfate formed in step (2) in a stream of hotcombustion gas to form ammonia and ammonium bisulfate; and (4) reactingthe ammonia with a second portion of the absorber effluent solution toprepare fresh absorbent solution. The principal reactions taking placein these four steps are as follows:

The process will now be described in greater detail with reference tothe accompanying drawing.

A flue gas stream 10 containing sulfur dioxide is introduced into aquench cooler and scrubber ll,'where the up-flowing flue gas stream isquenched by contact with a down-flowing stream of water introducedthrough inlet 12. This operation cools andhumidifles the flue gas streamand removes solid particles, such as fly ash, which are present in smallamounts. Waste water is removed from the bottom of the scrubber viaoutlet 13. This water may be neutralized and conveyed to a settling pondto permit fly ash to settle out. The flue gas stream, which issubstantially saturated with water vapor, is removed through overheadconduit I4.

,The flue gas stream flows continuously from overhead line 14 into thebottom of sulfur dioxide absorber 20. This absorber is illustratedherein as a packed tower which containsa conventional packing materialsuch as Raschig rings. Fresh absorbent solution, which is essentially amixture of aqueous ammonium sulflte and aqueous ammonium bisulfitehaving a pH greater than 6 but less than 7, is fed from absorbentsolution surge tank 30 to absorber 20 via absorbent solution feed lines31a, 31b and Me. The use of ammonium sulflte instead of ammonia as theabsorbent minimizes loss of ammonia to the atmosphere. Ammonia lossesare further reduced by splitting the absorbent solution feed intothree'portions 31a, 31b and Me, instead of a'single feedstream enteringthe top of absorber 20. Makeup ammonia, to compensate for losses ofammonia from the system, may be introduced through ammonia makeup inletpipe 32. The absorbent solution flows down through the tower 20countercurrent to the flue gas flow, and the entire absorber effluentsolution is withdrawn from the bottom of the tower through line 33. Thisabsorber effluent solution is principally ammonium bisulfite.Desulfurized flue gas is withdrawn through flue gas outlet 34, whichleads to a stack for discharge of gas to the atmosphere. Thedesulfurized flue gas typically has a sulfur dioxide content no greaterthan 10 percent of the original sulfur dioxide content of the enteringflue gas.

In order to promote better contact between flue gas and absorbentsolution, and to keep the packing wet throughout the tower, it isfrequently desired to recirculate a portion of the absorbent solution.Recirculation lines for this purpose have been omitted from theaccompanying drawing for the sake of clarity. For recirculation ofabsorbent liquid, it is desirable to divide the tower 20 into aplurality of stages by means of trays not shown) which will permit fluegas to pass upwardly through the tower freely but collect thedownflowing absorbent solution, and to recirculate a portion of theabsorbent solution collected on each tray to the top of the stage whiletank 30 into the upper portion of each stage. This mode of operation isfurther described in the copending application of Lindsay l. Griffin,Jr., Ser. No. 869,225, now abandoned filed of even date herewith.

Instead of the absorption tower 20, other multiple-stage countercurrentgas-liquid contact devices, such as a series of venturi scrubbers, maybe used.

The fresh absorbent solution, or lean solution, in surge tank 30contains from about 1 l to about 17 moles of ammonia per 100 moles ofwater and from about 6.5 to about 10 moles of sulfur dioxide per 100moles of water. The absorber effluent solution, a rich solution, ineflluent line 33 is primarily aqueous ammonium bisulflte with someammonium sulfite present;

the composition ranges from about 11 to about 18 moles of NH per 100moles of water, and from about 810 about 16 moles of SO per 100 moles ofwater. The effluent solution preferably contains from about 12 to 16moles of ammonia and from about 10 to 14 moles of SO per 100 moles ofwater.

As the flue gas rate in absorber 20 varies, the absorbent solution rateis varied proportionately so that the abovedescribed absorbent andabsorbereffluent solution compositions can be maintained. Considerablevariation in the flue gas rate normally occurs in the course of a day atan electric power plant because of varying power requirements.

The absorber 20 is operated at a temperatureof about 30 C. (86 F.) toabout 75 C. (167 F.) and from substantially atmospher'ic pressure up toabout atmospheres. Preferred operation is from about 35 C. (95 F.) toabout 60 C. (140 F Pressures over about 5 atmospheres can be used, butthe advantages of reduced equipment size and greater solubility ofsulfur dioxide are generally outweighed by the disadvantages ofhigh-pressure operation, such as the need for high-pressure equipment.Operation of absorber 20 is essentially isothermal.

The absorber effluent solution which is withdrawn through line 33(except for a portion which may be recirculated within the lowest stage20a through recirculation line not shown) is continuously introducedinto an absorber effluent solution surge tank 40. Naturally, the rate ofintroduction of solution into this tank will be highest at peak loadtimes, since the rate at which fresh solution is introduced intoabsorber 20 is proportional to the flue gas flow rate through absorber20. In this way, a substantially constant composition of absorbereffluent solution is maintained, regardless of variations in load.

The present invention permits the regeneration of the absorber effluentsolution to be carried out at a constant flow rate, regardless of thevariations of the solution flow rate within the absorber 20. To thisend, solution is withdrawn at a constant rate from effluent surge tank40.

The absorber effluent solution is withdrawn from surge tank 40 throughline 41, and is divided into two portions. The first and smallerportion, stream 42, is treated to liberate sulfur dioxide. The secondand larger portion, stream 43, is used to prepare fresh absorbentsolution.

The absorber effluent solution in line 42 is introduced into anacidifier 50. An excess of ammonium bisulfate is introduced in themolten state into acidifier 50 via line 51. The acidifier 50 ispreferably operated at a temperature of about 200 F. to 225 F., and theheat released in the acidifier is supplied principally by the heatcontent of the molten ammonium bisulfate. Sulfur dioxide is liberated inacidifier 50 by the reaction of ammonium bisulfite and ammoniumbisulfate. An appreciable quantity of water is vaporized. A mixture ofsulfur dioxide and steam leaves the acidifier 50 via vapor exit line 52.The sulfur dioxide in line 52 may be further processed in order torecover either sulfur or sulfuric acid. When sulfur is desired as abyproduct, the sulfur dioxide-steam mixture in line 52 may be conveyeddirectly to a Claus plant. When sulfuric acid is desired, the watervapor in line 52 may be separated from the sulfurdioxide by conventionalmeans, and the dry SO; ox-

idized to S0, and converted to sulfuric acid by known means.

Liberation of sulfur dioxide in acidifier 50 results in the formation ofammonium sulfate as a byproduct. Since substantial quantities of waterare vaporized, this ammonium sulfate is in the form of an aqueousslurry, which may also contain some unreacted ammonium bisulfate. Thisslurry has a total solids content (dissolved plus undissolved salts) ofabout 60 to about percent by weight. The undissolved solids constituteabout 15 to 30 percent by weight of the total slurry. Higherconcentrations are generally not desirable because they are not fluidenough for easy pumping; lower concentrations are generally not desiredbecause the subsequent decomposition of am monium sulfate would requireexcessive amounts of heat to vaporize the water.

The slurry of ammonium sulfate is removed from acidifler 50 through line53, and is introduced into decomposer 60. The ammonium sulfate slurry isintroduced into a stream of hot combustion gases which enter thedecomposer 60 from combustion gas inlet line 61. These combustion gasesare formed by the combustion of a fossil fuel such as natural gas, fueloil, or coal. The decomposer 60 is operated at a temperature of about600 F. to about 900 F., preferably about 700 F. to about .850 F. At thistemperature ammonium sulfate is decomposed into molten ammoniumbisulfate and ammonia. Temperatures substantially in excess of about 900F; are avoided since such temperatures would cause further decompositioninto sulfur trioxide and additional ammonia, and may even cause somenitrogen and sulfur dioxide to be formed. Molten ammonium bisulfate iswithdrawn from the bottom of decomposer 60 through line 51, from whichit is introduced into acidifier 50 as previously described. Thedecomposer 60 is preferably ceramic lined, since the molten salt thereinis quite corrosive. In a preferred mode of operation, decomposer 60 isplaced at a higher level than acidifier 50, so that the molten ammoniumbisulfate in line 51 can flow by gravity, thus obviating the necessityfor high-temperature pumps. A gas stream comprising ammonia andcombustion gases (i.e., nitrogen, carbon dioxide, and water vapor) iswithdrawn from decomposer 60 through overhead line 62. This stream iscooled in cooler 63 prior to removal of the ammonia therefrom.

Cool ammonia-containing gas from line 62 is contacted with absorbereffluent solution from line 43 in ammonia absorber 10, in order to makefresh absorbent solution. Absorber 70 may be a conventional packed towerin which the ammonia gas stream and the absorber effluent solutionstream flow in countercurrent contact. Additional water is added toabsorber 70 as required through line 71; the diluted absorber effluentsolution is added to absorber 70 through liquid inlet 72. The ammoniaabsorber 70 is operated at about the same temperature as the sulfurdioxide absorber 20, Le, in the range of about 30 C. (86 F.) to about 75C. (167 F. and preferably at about 35 C. F.) to about 60 C. F.). Freshabsorbent solution for use in the sulfur dioxide absorber 20 iswithdrawn from the bottom of the ammonia absorber 70 through absorbentsolution outlet line 73, and is returned to the fresh absorbent solutionsurge tank 30. A portion of this solution may be recirculated to the topof the ammonia absorber 70 in order to improve gas-liquid contacttherein.

The gases from line 62 which are not absorbed in ammonia absorber 70,such as nitrogen and carbon dioxide, are removed from the absorber 70through overhead line 74. These gases may be introduced into the upperportion of sulfur dioxide absorber 20 and thence vented through overheadline 34 into a stack and thence to the atmosphere.

It will be seen that the acidifier 50, the decomposer 60, and theammonia absorber 70 are operated at substantially constant flow rates,even though the flow rate of both flue gas and absorbent liquid inabsorber 20 may vary widely. This is made possible by the use ofammonium sulflte rather than gaseous ammonia as the absorbent inabsorber 20, and by placing surge tanks 30 and 40 for fresh absorbentsolution and ab sorber effluent solution, respectively, between thesulfur dioxide absorber 20 and the rest of the system. Ammonium sulfitecan be conveniently stored in surge tank 30 in aqueous solution, whilegaseous ammonia would have to be used as it is produced. in times ofheavy gas flow through absorber 20, as, for example, when an electricpower generating plant which releases flue gas is operating at capacity,fresh solution is withdrawn from tank 30 faster than it is suppliedthereto, and effluent solution flows into tank 40 faster than it iswithdrawn therefrom. Conversely, at times of low flow rate in absorber20, e.g., during a low load period in an electric power generatingplant, effluent solution is withdrawn from tank 40 greater than itenters therein, and the supply of fresh solution in tank 30 isreplenished.

A further advantage of the regeneration system described herein is itslow heat requirements. The amount of heat necessary to regenerate theabsorbent solution according to this invention is only about 2 percentof the amount of heat liberated in the furnace where the flue gas isformed. The instant regeneration process requires substantially lessheat than does boiling of the absorber effluent to recover sulfurdioxide.

To minimize corrosion, it is preferred to use equipment which is made ofcorrosion resistant alloy, or which is ceramic lined where severecorrosivity is encountered, as, for example, in decomposer 60.

This invention will now be described further with reference to aspecific example.

The stream quantities in this example are based on the assumption that90 percent of the sulfur dioxide content in the flue gas is removed andthat equilibrium is reached in all stages of the system. Actually,desulfurization of greater than 90 percent, even as high as 95 percentor more, can be achieved. On the other hand, equilibrium is never fullyattained. The stream quantities used in the example which follows aretypical of those to be encountered in the desulfurization of the gluegas stream from an 800-megawatt electric power generating plantoperating at capacity. EXAMPLE Flue gas containing a small quantity ofsulfur dioxide is scrubbed with water vapor in quench cooler 11.Scrubbing quench cools the gas stream and substantially saturates thestream with water vapor. The flue gas stream is passed upwardly throughsulfur dioxide absorber 20, where it is contacted with a downflowingstream of aqueous absorbent solution. This solution is essentiallyammonium sulfite. with some ammonium bisulftte present, containingtypically about 14.0 moles of NH and 8.2 moles of SO per 100 moles ofwater and having a pH of 6.5. This absorbent solution is introduced atthree points in the absorber 20 via lines 31a, 31b and 310. Makeupammonia is added through line 32. Absorber 20 is operated at 122 F. andsubstantially atmospheric pressure. Desulfurized flue gas is withdrawnoverhead via line 34, and

sorber 70, forming absorbent solution which is primarily am moniumsulfite. This solution is passed from absorber 70 to surge tank 30 vialine 73. Solution is withdrawn from surge absorber effluent solution iswithdrawn through line 33 and passed to surge tank 40. The absorbereffluent solution is predominantly ammonium bisulfite containingtypically about l4.0 moles of NH and 11.5 moles of SO per 100 moles ofwater, and having a pH of5.4.

Absorber effluent solution is withdrawn from surge tank 40 via line 41,and is divided into two streams 42 and 43. Stream 42 flows to acidifier50, and molten ammonium bisulfate is also introduced into acidifier 50via line 51. The solution temperature in acidifier 50 is about 200 F.Sulfur dioxide and water vapor are removed overhead via line 52.

An aqueous slurry of ammonium sulfate, with some ammonium bisulfate,containing typically about 15 percent by volume of solids, flows fromacidifier 50 to decomposer 60 via line 53. The ammonium sulfate slurryis contacted in decomposer 60 with a hot combustion gas stream enteringthrough line 61, forming molten ammonium bisulfate, which is withdrawnfrom the decomposer via line 51 and returned to the acidifier 50. Theammonia is entrained in the combustion gas exit stream 62, which flowsto the ammonia absorber 70.

Ammonia from line 62 and ammonium bisulfate solution from line 43 arecountercurrently contacted in ammonia abtank 30 via lines 31a, 31b and31c, and introduced into the sulfur dioxide absorber as previouslydescribed. Combustion gas is withdrawn from absorber 70 via overheadline 74.

Stream quantities in pound moles per hour are shown in Table l below.

TABLE I Stream quantities in pound moles per hour. Reference numeralsrefer to drawing.

l Total flow, including S0,.

(2) Essentially saturated.

(3) Combustion gas.

(4) Total flow, including water vapor.

(5) Includes water vapor formed by combustion, but does not include N Hor water introduced in stream 53.

(6) Includes water from stream 53 only. Does not include water vapor incombustion gas (stream 6| (7) Water is added as required.

Table ll below shows the stream quantities (in pound moles per hour) instreams 51 and 53.

TABLE II Stream Constituent Si 53 Ammonium bisulfate Ammonium sulfate insolution Ammonium sulfate in suspension Water Notes:

( I Molten.

(2) In solution.

While this invention has been described with particular reference toflue gas, other gases containing sulfur dioxide, such as smelter gasfrom pyrites and other ore-roasting processes, can also be desulfurizedaccording to the present invention.

l. A process for removing sulfur dioxide from flue gas which comprises:

a. introducing a stream of flue gas containing sulfur dioxide into anabsorption zone;

b. withdrawing a fresh aqueous absorbent solution containing ammoniumsulfate and ammonium bisulfite and having a pH of about 6 to about 7from a first tank and introducing said solution into said absorptionzone; I

c. contacting said flue gas stream and said absorbent solution in saidabsorption zone;

d. withdrawing a flue gas stream of substantially reduced sulfur dioxidecontent and an effluent solution containing ammonium bisulfite as theprincipal solute from said absorption zone and introducing effluentsolution into a second tank;

e. withdrawing said effluent solution from said second tank at asubstantially constant rate and dividing said effluent solution into twoportions;

acidifying the first portion of said absorber effluent solution withammonium bisulfate, thereby liberating sulfur dioxide and formingammonium sulfate;

g. decomposing said ammonium sulfate at elevated temperature intoammonium bisulfate and gaseous ammonia in a decomposition zone;

h. reacting said gaseous ammonia with the second portion of 25 saidabsorber effluent solution to prepare fresh aqueous absorbent solution;and

. returning said fresh aqueous absorbent solution to said first tank. Iv

2. A process according to claim 1 in which said ammonium bisulfate instep (f) is 'molten, whereby a substantial quantity of water isvaporized and said ammonium sulfate is formed as an aqueous slurry. I

3. A process according to claim 2 in which said aqueous slurry containsfrom about l5 to about 30 percent by weight of undissolved solids.

4. A process according to claim 1 in which said ammonium sulfate isdecomposed at a temperature in the range of about 600 F. to about 900 F.

5. A process according to claim 1 in which said ammonium sulfate isdecomposed at a temperature in the range of about 700 F. to about 850 F.

6. A process according to claim 2 in which said aqueous slurry isintroduced into said decomposition zone and ammonium sulfate therein isdecomposed by contact with hot combustion gases.

7. A process according to claim 1 in which said flue gas stream and saidabsorbent solution flow at variable rates in said absorption zone, andin which the flow rates in steps (F), (g), and (h) are substantiallyconstant.

8. A process according to claim 1 in which said fresh absorbent solutionis introduced into said absorption zone at a rate which is substantiallyproportional to the rate of flue gas flow through said absorption zone.

2. A process according to claim 1 in which said ammonium bisulfate instep (f) is molten, whereby a substantial quantity of water is vaporizedand said ammonium sulfate is formed as an aqueous slurry.
 3. A processaccording to claim 2 in which said aqueous slurry contains from about 15to about 30 percent by weight of undissolved solids.
 4. A processaccording to claim 1 in which said ammonium sulfate is decomposed at atemperature in the range of about 600* F. to about 900* F.
 5. A processaccording to claim 1 in which said ammonium sulfate is decomposed at atemperature in the range of about 700* F. to about 850* F.
 6. A processaccording to claim 2 in which said aqueous slurry is introduced intosaid decomposition zone and ammonium sulfate therein is decomposed bycontact with hot combustion gases.
 7. A process according to claim 1 inwhich said flue gas stream and said absorbent solution flow at variablerates in said absorption zone, and in which the flow rates in steps (F),(g), and (h) are substantially constant.
 8. A process according to claim1 in which said fresh absorbent solution is introduced into saidabsorption zone at a rate which is substantially proportional to therate of flue gas flow through said absorption zone.